Castor JDO Key Generator Support

The key generator gives a possibility to generate identity field
values automatically. During create the value of the
identity field is set to the value obtained from the key generator.
Different algorithms may be used here, both generic and specific
for database server.

The key generator for the given class is set in the mapping
specification file (mapping.xml), in the
key-generator attribute of the class element,
for example:

MAX key generator fetches the maximum value of the primary key
and lock the record having this value until the end of transaction.
Then the generated value is set to (MAX + 1).
Due to the lock concurrent transactions which perform insert
to the same table using the same key generator algorithm will wait
until the end of the transaction and then will fetch new MAX value.
Thus, duplicate key exception is almost impossible (see below).
Note, that it is still possible to perform multiple inserts during
the same transaction.

There is one "singular" case of this algorithm: the case when
the table is empty.
In this case there is nothing to lock, so duplicate key exception
is possible.
The generated value in this case is 1.

This algorithm has no parameters.
Primary key must have type integer, bigint or numeric.

This key generator uses one of the variants of the generic HIGH-LOW
algorithm. It is needed a special auxiliary table ("sequence table")
which has the unique column which contains table names (of the type
char or varchar) and the column which is used to
reserve values of the primary keys (of the type integer,
bigint or numeric).
The key generator seeks for the given table name, reads the last
reserved value and increases it by some number N, which is called
"grab size". Then the lock on the auxiliary table is released,
so that concurrent transactions can perform insert to the same table.
The key generator generates the first value from the grabbed
interval.
During the next (N - 1) invocations it generates the other grabbed
values without database access, and then grabs the next portion.
Note, that the auxiliary table must be in the same database as
the table for which key is generated. So, if you work with multiple
databases, you must have one auxiliary table in each database.

If the grab size is set to 1, the key generator each time stores
the true maximum value of the primary key to the auxiliary table.
In this case the HIGH-LOW key generator is basically equivalent to
the MAX key generator.
On you want to have LOW part of the key consisting of 3 decimal
digits, set the grab size to 1000. If you want to have 2 LOW bytes in
the key, set the grab size to 65536.
When you increase the grab size, the speed of
key generation also increases because the average number of
SQL commands that are needed to generate one key is (2 / N).
But that average number of key values that will be skipped (N / 2)
also increases.

The HIGH-LOW key generator has the following parameters:

table

The name of the special sequence
table.

Mandatory

key-column

The name of the column which contains
table names

Mandatory

value-column

The name of the column which is used to
reserve primary key values

Mandatory

grab-size

The number of new keys the key
generator should grab from the sequence table at a time.

Optional, default="10"

same-connection

To use the same Connection for writing to the sequence
table, values:
"true"/"false". This is needed when working in EJB environment,
though less efficient.

Optional, default="false"

global

To generate globally unique keys, values:
"true"/"false".

Optional, default="false"

If the parameter "global" is set to "true", the sequence table contains only one row with
"<GLOBAL>" instead of the table name. This row serves for all tables.

Don't forget to set same-connection="true" if you are working in EJB environment!

Note, that the class HighLowKeyGenerator is not final, so you can
extend it in order to implement other variants of HIGH-LOW
algorithm (for example, HIGH/MID/LOW or char key values).

This key generator generates global unique primary keys.
The generated key is a combination of the IP address, the current
time in milliseconds since 1970 and a static counter.
The complete key consists of a 30 character fixed length string.

This algorithm has no parameters.
Primary key must have type char, varchar or
longvarchar.

IDENTITY key generator can be used only with autoincrement primary
key columns (identities) with Sybase ASE/ASA, MS SQL Server, MySQL and
Hypersonic SQL.
After the insert the key generator selects system variable
@@identity which contains the last identity value for
the current database connection.
In the case of MySQL and Hypersonic SQL the system functions
LAST_INSERT_ID() and IDENTITY() are called, respectively.

SEQUENCE key generator can be used only with Oracle, PostgreSQL,
Interbase and SAP DB.
It generates keys using sequences.

The key generator has the following parameters:

sequence

The name of the sequence

Optional, default="{0}_seq"

returning

RETURNING mode for Oracle8i, values:
"true"/"false"

Optional, default="false"

increment

Increment for Interbase

Optional, default="1"

trigger

Assume that there is a trigger that already
generates key. Values: "true"/"false"

Optional, default="false"

Usually one sequence is used for one table, so in general you have to
define one key generator per table.
But if you use some naming pattern for sequences, you can use one
key generator for all tables.
For example, if you always obtain sequence name by adding "_seq" to
the name of the correspondent table, you can set "sequence" parameter
of the key generator to "{0}_seq" (the default value).
In this case the key generator will use sequence "a_seq" for table
"a", "b_seq" for table "b", etc. Castor also allows for inserting
the primary key into the sequence name as well. This is accomplished
by including the {1} tag into the "sequence" parameter. Example:
"{0}_{1}_seq"

Actually the SEQUENCE key generator is "4 in 1".
With PostgreSQL it performs "SELECT nextval(sequenceName)" before
INSERT and produces identity value that is then used in INSERT.
Similarly, with Interbase it performs
"select gen_id(sequenceName, increment) from rdb$database" before
INSERT.
With Oracle by default (returning=false) and with SAP DB it transforms
the Castor-generated INSERT statement into the form
"INSERT INTO tableName (pkName,...) VALUES
(sequenceName.nextval,...)" and after INSERT it performs "SELECT
seqName.currval FROM tableName" to obtain the identity value. With
Oracle8i it is possible to use more efficient RETURNING mode: to
the above INSERT statement is appened "RETURNING primKeyName INTO
?" and the identity value is fetched by Castor during INSERT, so
that only one SQL query is needed.

In case when your table has an on_Insert trigger which already generates
values for your key, like the following Oracle example:

create or replace trigger "trigger_name"
before insert on "table_name" for each row
begin
select "seq_name".nextval into :new."pk_name" from dual;
end;

you may set the "trigger" parameter to "true". This will prevent the
"Sequence_name".nexval from being pulled twice (first time in the insert
statement (see above), then in the trigger). Also usefull in combination
with the "returning" parameter set to "true" for Oracle (in this case you
may not specify the sequence name).

Java, EJB, JDBC, JNDI, JTA, Sun, Sun Microsystems are trademarks or registered
trademarks of Sun Microsystems, Inc. in the United States and in other
countries. XML, XML Schema, XSLT and related standards are trademarks or registered
trademarks of MIT, INRIA, Keio or others, and a product of the World Wide Web
Consortium. All other product names mentioned herein are trademarks of their respective
owners.